JPS6238380A - Measuring instrument using optical fiber - Google Patents

Abstract

PURPOSE:To reduce the size and weight of an instrument by supplying laser light emitted by a laser light source directly to an optical fiber whose end surface is tapered. CONSTITUTION:The laser light obtained from the semiconductor laser 11 is entered into the optical fiber from its tapered surface 10a through a lens 12 and projected from the other end surface 10b. Then, when a body 6 to be measured passes before it, reflected light is supplied to the optical fiber 10 again from the end surface 10b. This reflected light is Doppler-shifted (DELTAf) in proportion to the speed of the object body 6, so the frequency of the signal light is f0+DELTAf. Further, part of the laser light propagated in the optical fiber 10 is reflected as it is and propagated as reference light of frequency f0 in the optical fiber 10 in the opposite direction together with the signal light, and then reflected totally by the tapered surface 10a. A photoelectric converter 7 performs heterodyne detection by the reference light converged by a lens 13 and the light beam of the signal light to obtain an electric signal of frequency DELTAf. A frequency measuring instrument 8 measures the frequency DELTAf, which is processed by a signal processing part 9 to obtain speed information.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to a measuring device using an optical fiber, which measures the speed, etc. of an object using an optical fiber.

[Prior art and its problems]

For example, an optical fiber velocimeter that uses an optical fiber to measure the speed of an object uses the Doppler effect due to the optical beat of the laser light from the laser light source, such as laser light that is irradiated from the tip of the optical fiber to the object and reflected. Devices are known that measure the velocity of an object based on FIG. 6 shows an example of such a conventional optical fiber speed meter.

In this figure, laser light obtained from a laser light source 1 such as a He-Ne laser is applied to a polarizing beam splitter 3 via a lens 2. Assuming that the polarization of the laser light source 1 is adjusted in the direction in which the laser light passes through the polarizing beam splitter 3, the laser light passes through the polarizing beam splitter 3 and is applied to the lens 4. The laser beam is focused by a lens 4 and input into an optical fiber 5. A multi-mode optical fiber is used as the optical fiber 5, and the laser beam becomes randomly polarized when passing through the optical fiber.

The tip of the optical fiber 5 extends to a region through which the object to be measured passes, and the laser beam propagates through the optical fiber 5 and is emitted from the tip.

Now, when the object to be measured 6 approaches the tip of the optical fiber 5 as shown, the emitted laser light is reflected by the object to be measured 6 and is irradiated onto the optical fiber 5 again. - Unless anti-reflection coating is applied only to the tip end surface 5a of the destination fiber 5, the laser beam will be reflected also at the tip end surface 5a (hereinafter, this laser beam will be referred to as reference light). Here, only the light reflected by the object to be measured 6 undergoes a frequency shift (Doppler shift) in proportion to the speed of the object to be measured 6 due to the Doppler effect. Therefore, since this laser light contains a signal component, this laser light will hereinafter be referred to as signal light. Both the reference light and the signal light travel in opposite directions within the optical fiber 5 and are again applied to the polarizing beam splitter 3 via the lens 4. Since both the reference beam and the signal beam are randomly polarized, any of these laser beams with the same polarization component as the incident laser beam will pass through the polarization beam splitter 3, and the polarization component perpendicular to this will be transmitted through the polarization beam splitter 3. The laser beam having the polarizing beam splitter 3 is reflected by the polarizing beam splitter 3 and provided to the photoelectric converter 7. In the photoelectric converter 7, an electrical signal having a frequency based on the difference in frequency of these reflected lights, that is, the Doppler shift, is obtained by the optical beat. Therefore, the frequency of this signal is measured by the frequency measuring device 8.
・Based on the data, the signal processing unit 9
It becomes possible to measure the velocity of the object to be measured 6 by signal processing.

However, in such a conventional optical fiber velocimeter, the polarizing beam splitter 3 transmits the laser light oscillated from the laser light source 1 to the optical fiber 5 and provides the reflected light from the object to be measured to the photoelectric converter 7. is necessary. Moreover, since the polarization planes of the signal light and the reference destination are random, only a portion of the polarization plane is reflected by the polarization beam splitter 3 and transmitted to the photoelectric converter 7, resulting in a problem that the signal level becomes low. Also, the reference light and signal light that return to the laser light source 1 via the polarized beam splinter 3 are returned to the laser light source 1.
There was a problem in that the oscillation intensity, frequency, etc. of the laser beam fluctuated. In particular, it is conceivable to use a semiconductor laser as a laser light source in order to downsize the entire device.
Since the oscillation mode of a semiconductor laser is significantly affected by the return light of the laser beam, an expensive isolator must be installed between the laser light source 1 and the lens 2 to prevent the return light of the laser beam, making the device complicated. There was a problem with becoming.

[Purpose of the invention]

The present invention has been made in view of the problems of conventional measuring devices using optical fibers, and aims to reduce the size and weight of the device by applying laser light emitted from a laser light source directly to the optical fiber. The object of the present invention is to provide a measuring device using an optical fiber that can perform the following steps.

[Structure and effects of the invention]

A measuring device using an optical fiber according to the present invention has one end face inclined at a predetermined taper angle from the perpendicular line of the optical axis to form a tapered surface. an optical fiber whose angle is selected such that the light beam reaching the tapered surface from inside the fiber is totally reflected on the tapered surface, and whose other end surface is guided to the measurement area; a laser light source that is arranged at a predetermined angle with respect to the tapered surface of the optical fiber and makes laser light enter the optical fiber;
A photoelectric converter that receives the totally reflected light of the laser beam that passes through the optical fiber, is reflected by the object to be measured, propagates in the opposite direction through the optical fiber, and reaches the tapered surface of the optical fiber, and converts the optical signal into an electrical signal. , and a signal processing section that performs signal processing based on the output of the photoelectric converter.

According to the present invention having such characteristics, by processing the end face of the optical fiber into a tapered shape, it is possible to separate the incident light and the outgoing light without using expensive parts such as a polarizing beam splitter. It is. Further, since the reflected light is totally reflected by the tapered surface of the optical fiber, there is no return light returning to the laser light source, and there is no adverse effect on the oscillation of the laser light. It is possible to use a semiconductor laser as a laser light source without providing an isolator or the like.

Therefore, in addition to eliminating the need for a polarizing beam splitter, it is possible to make the device extremely compact and lightweight.

[Explanation of Examples]

(Principle of the invention) Next, the principle of the invention will be explained. As shown in Fig. 2, the measuring device using the optical fiber of the present invention cuts the end face of the optical fiber obliquely, makes the laser beam enter the cut face at an acute angle, and outputs the reflected light from the cut face. This is what I did. In FIG. 2, the refractive index of the core of the optical fiber 10 is nl+, the refractive index of the cladding is 02, and the angle at which the end face of the optical fiber 10 is cut (hereinafter referred to as the Taber angle) is α
shall be.

Here, in order to consider the incident characteristics of this optical fiber 10, an appropriate incident angle is determined for the taper angle α. Normally, the allowable angle of incidence of an optical fiber is determined by the numerical aperture NA. It is known that the following relationship holds between the numerical aperture NA and the internal critical angle θc of the optical fiber.

NA=n, sinθc #aIX n, -・-
= (1) where Δ is the relative refractive index difference (nt - nx)
/n+. From this formula, the critical angle θc can be found from the refractive index nl and NA. For example, the relative refractive index difference Δ is 0.01
．． If the numerical aperture NA is 0.2, the refractive index n1 is 1.41
．． The critical angle θc is approximately 8.2°.

Light beams having respective angles of θ1° and θ2 with respect to the normal line of the tapered surface are incident on the center of the core of the tapered surface 10a (7) of this optical fiber 1o, and within the core of the optical fiber 10, each of the light beams has an angle of α- Assuming that the movement progresses in a direction that satisfies θc1α+θc, the following equation holds true from Snell's law.

However, θl, θ2<π/2 Therefore, the incident angle to the optical fiber 10 whose end face is processed as the tapered surface 10a must be a light beam between the angles θ1 and 02, and the allowable incident angle θi(−θ
2-01) is expressed by the following formula.

θi = sin-'(n+5in(α+θc))-si
n-'(n+5in(&-θc)) --------=
(3) And this taper angle α is 10', 30°, 45"
When the allowable incident angle θi is shown in FIG. 3(a),
(b), ' changes as shown in (C)=Then, when the taper angle α is about 45'', a part of the allowable incident angle θi coincides with the tapered surface 10a of the optical fiber 1o, so from now on, the allowable incident angle θi becomes Therefore, the change in allowable incident angle θi with respect to taper angle α is as shown in Figure 4, and when the taper angle is 3
It rises rapidly in the vicinity of 0° to 45＠ and then decreases.

The critical angle at which the light beam cannot enter the optical fiber 10 is when the incident angle θ1 coincides with the tapered surface 10a, that is, when θ1 is 90@,
), the limit angle of the taper angle α is determined by the following equation.

α-5in(1/nυ+θc For example, in the above-mentioned optical fiber, the critical angle is 53.37''.

Next, referring to FIG. 5, the tapered surface 10 of the optical fiber 10
Let us consider the emission characteristics from a. As shown in the figure, the light passing through the core of the optical fiber 10 has a reflection angle that is less than the critical angle θc. This transmitted light is transmitted through the optical fiber 10.
The conditions for total reflection at the tapered surface 10a are expressed by the following equation.

θ3 > 5in (1/n+) θ3-α-θc ------441.

Here, if the refractive index n of the core part is 1.41, α>5
3.37 The conditions under which total reflection occurs at 4° and no reflection at all at the tapered surface 10a are expressed by the following equation.

θ4 > 5in (1/n+) θ4−α+θc−・−−−−−−(5) Therefore, the critical angle is the following formula (%), and in this optical fiber, reflection does not occur when α< 36.97°. There will be no. And the taper angle α is 5
Between the angles in(1/nt) - θc and 5in(1/n1) + θc (in this example, 36.97°<α< 53.37
), the output range angle θ0 decreases linearly. Therefore, the change in the output angle θ0 with respect to the taper angle α is as shown in FIG. Figure 4 shows the incident angle θi.

The light enters and exits the tapered surface 10a of the optical fiber 10 within the range of the taper angle α where the output angles θc overlap.

(Configuration of Example) Next, an example of the measuring device using the optical fiber of the present invention will be described. FIG. 1 is a block diagram showing an embodiment of an optical fiber velocimeter according to the present invention. In this embodiment, a semiconductor laser 11 is used as a laser light source, and the laser light is focused by a lens 12 and applied to an optical fiber 10 having a tapered surface 10a. As described above, this taper angle α is within the range where the allowable incident angle θi and the output angle θ0 overlap, for example, in the example of the optical fiber described above, from 36.97' to 53.9'.
The taper angle α (approximately 51＠) shall be selected between 37°, preferably at which the two curves in FIG. 4 intersect. At this time, it is assumed that the semiconductor laser 11 is arranged at an angle such that the laser beam is incident into the optical fiber 10 from the taper angle α of the optical fiber 10. Then, the other end portion 10b of the optical fiber 10 is placed in the region to be measured. - In order to receive the light that is totally reflected by the tapered surface 10a of the destination fiber IQ and exits to the outside via the cladding, a condenser lens is placed on the optical axis having a larger inclination than the tapered surface 10a as shown in the figure. A photoelectric converter 7 is provided via 13. Similar to the conventional example, a frequency measuring device 8 that measures the frequency of the electrical signal obtained from the photoelectric converter 7 and a signal processing section 9 that calculates the velocity of the object to be measured based on the measured value are provided.

(Operation of this embodiment) Next, the operation of this embodiment will be explained. Semiconductor laser 1
1 is applied to the tapered surface 10a of the optical fiber 10 via the focusing lens 12. Tapered surface 10
The light enters the optical fiber 10 from the point a, is transmitted to the region to be measured, and is emitted from the other end face 10b. Now, the object to be measured 6 is placed in front of the end face 10b of the optical fiber 10 in the area to be measured.
When the laser beam passes through, the reflected light from this laser beam is given to the optical fiber 10 again from the end face 10b.

Since this reflected laser light undergoes a Doppler shift (Δf) proportional to the speed of the object to be measured 6, the frequency of the signal light becomes fo+Δf. Also, the end face 10 of the optical fiber 10
Since no anti-reflection coating is applied to beam b, a part of the laser beam propagated through the optical fiber 10 is directly reflected and propagates in the opposite direction through the optical fiber 10 together with the signal beam as a reference beam having the frequency fO. and tapered surface 10a
When reaching this point, the signal light and the reference light are totally reflected on the tapered surface 10a as described above, and are focused by the condenser lens 13 and provided to the photoelectric converter 7. In the photoelectric converter 7, heterodyne detection is performed using the light beams of the reference light and the signal light, and an electrical signal having a frequency Δf based on the difference in the frequencies of these laser lights, that is, the Doppler shift is obtained. The frequency measuring device 8 measures the frequency (Δf) of this signal, and the signal processing section 9 processes the obtained data to obtain speed information.

Although the embodiment has been described here as an apparatus for measuring the instantaneous velocity of an object to be measured, it can also be used as an apparatus for measuring the vibration of an object by continuously measuring the instantaneous velocity.

In addition to devices that measure the speed and vibration of objects, the present invention can be applied to various devices that use an optical fiber to pass laser light in both directions and separate the output light and the input light at the end face of the optical fiber. Is possible.

[Brief Description of the Drawings] Fig. 1 is a block diagram showing an embodiment of a measuring device using an optical fiber according to the present invention, and Fig. 2 shows an input of an optical fiber whose end face is tapered. Principle diagram showing the state,
Fig. 3(a), Fig. 3(b), and Fig. 3(C) are diagrams showing changes in the allowable incident angle with respect to the taper angle α, and Fig. 4 shows the allowable incident angle θi and the exit angle with respect to the taper angle α. FIG. 5 is a graph showing the change in θO, FIG. 5 is a principle diagram showing the state of emission from the tapered surface of an optical fiber, and FIG. 6 is a block diagram showing the configuration of a speedometer using a conventional optical fiber. 1.------Laser light source 2. 4. 12-・
-Lens 3--Polarizing beam splitter 5.
10...--Optical fiber 10a--
Tapered surface 6−・・−・Object to be measured 7−・−・−
Photoelectric converter 8 --- Frequency measuring device 9 ---
--- Signal processing unit 11 --- Semiconductor laser 13 --- Condensing lens Patent applicant Nihon Kagaku Kogyo Co., Ltd. Patent attorney Kanki Okamoto (and one other person) Figure 1 1o・---・---Ichisaki Fufui Igu 10a----・Chee lug side 10b---- 97 Tei I < pigeon product 11---・---
--Ichihan 4 Yashiichisu゛Fig. 2 1゜Fig. 3 0a 0a Fig. 4 36.97 53.37 Chi Igu^ bi 0 Fig. 5 0a

Claims (3)

[Claims] (1) One end face is configured as a tapered surface by tilting a predetermined taper angle from the perpendicular line of the optical axis, and the taper angle is such that a light beam enters from the outside through the tapered surface and reaches the tapered surface from inside the optical fiber. The angle is selected such that the optical beam is totally reflected on the tapered surface, and the other end surface is placed at a predetermined angle with respect to the optical fiber guided to the measurement area and the tapered surface of the optical fiber, and the laser beam is emitted. a laser light source that is incident on the optical fiber; and a system that receives totally reflected light of the laser light that passes through the optical fiber, is reflected by the object to be measured, propagates in the opposite direction through the optical fiber, and reaches the tapered surface of the optical fiber. A measuring device using an optical fiber, comprising: a photoelectric converter that converts an optical signal into an electrical signal; and a signal processing section that processes a signal based on the output of the photoelectric converter. (2) The measuring device using an optical fiber according to claim 1, wherein the laser light source is a semiconductor laser. (3) The taper angle α of the optical fiber is sin when the refractive index of the core of the optical fiber is n_1 and the critical angle is θc.
(1/n_1)-θc to sin(1/n_1)+θc
A measuring device using an optical fiber according to claim 1, wherein the measuring device is selected between the following.